1. Field of the Invention
The present invention relates to a motion control apparatus for a four-wheel drive vehicle in which drive force of a drive source is transmitted to front and rear wheels. In particular, the present invention relates to a motion control apparatus that performs idle-rotation suppression control for the front and rear wheels when the difference in wheel speed between left and right wheels exceeds an allowable limit, so as to impart braking force for suppressing idle rotation to the wheel of the left and right wheels that is of the higher wheel speed, and also performs vehicle stabilization control which imparts braking force to the front wheel located on the outer side of a turning locus in order to generate a yawing moment in a direction opposite the vehicle's yawing direction when the vehicle is in a state of over-steer.
2. Description of the Related Art
Conventionally, there has been widely known a vehicle motion control apparatus that performs the above-described vehicle stabilization control (over-steer suppression control) in order to maintain running stability of the vehicle during turning (see, for example, Japanese Patent Application Laid-Open (kokai) No. 2004-66941). Specifically, a vehicle motion control apparatus of such a type determines that the vehicle is in a state of over-steer, for example, when the difference between the actual yaw rate obtained from a yaw rate sensor and a yaw rate calculated from steering angle (turn angle of the steering wheels), vehicle body speed, specifications of the vehicle, etc., exceeds a predetermined threshold value.
Which the vehicle is determined to be in an over-steer state, in general, the apparatus of such a type imparts a predetermined braking force, by means of brake hydraulic pressure, to the front wheel located on the outer side of a turning locus in order to generate a yawing moment (over-steer suppressing moment) in the vehicle in a direction opposite the vehicle's yawing direction. In addition, in order to reduce the centrifugal force acting on the vehicle, the apparatus reduces the output of the engine by a predetermined amount to thereby lower the vehicle body speed.
Meanwhile, there has also been widely known a vehicle motion control apparatus which performs the above-described idle-rotation suppression control in order to secure the running performance (performance of running through mud or the like) and escaping performance (performance of escaping from mud or the like) of the vehicle, in particular, on unpaved roads (see, for example, PCT Application Publication No. W091/04895). Specifically, when the difference in wheel speed between left and right wheels (driven wheels) obtained from, for example, wheel speed sensors, exceeds an allowable limit, the motion control apparatus of such a type imparts a predetermined braking force, by means of brake hydraulic pressure, to the one of the left and right wheels that is of higher wheel speed. This secures sufficient distribution of the engine output (torque) to the wheel to which the above-described braking force is not imparted. As a result, the running performance and the escaping performance of the vehicle are secured.
In recent years, there has been demand for maintaining the running stability of a vehicle during turning, while securing the running performance and the escaping performance of the vehicle. Such demand is strong in particular for four-wheel drive vehicles (in which drive force of a drive source is transmitted to front and rear wheels (i.e., both the front and rear wheels are driven wheels). In order to meet the demand, the four-wheel drive vehicle may be equipped with a motion control apparatus which performs the above-mentioned idle-rotation suppression control for both the front wheels and the rear wheels, as well as the above-mentioned vehicle stabilization control.
Incidentally, in general, when the output of an engine mounted on a vehicle once decreases, a relatively long time is required for the output to return to the level before the decrease. As a result, the running performance and the escaping performance of the vehicle lower over a period between a time when the output of the engine decreases and a time when the output returns to the original level. In other words, reducing the engine output for the above-mentioned vehicle stabilization control results in a decrease in the running performance and the escaping performance of the vehicle.
Accordingly, when the motion control apparatus which performs the above-mentioned idle-rotation suppression control for both the front wheels and the rear wheels and also performs the above-mentioned vehicle stabilization control is applied to a four-wheel drive vehicle in order to prevent a decrease in the running performance and the escaping performance of the vehicle, it is conceivably desired to impart a predetermined braking force to the front wheel located on the outer side of a turning locus, without reducing the engine output for the above-mentioned vehicle stabilization control.
However, in such a case, the following problems arise. Here, a case is considered in which a driver demands a higher engine output when the vehicle stabilization control is being performed; i.e., a predetermined braking force is being imparted to the front wheel located on the outer side of a turning locus. In this case, the high engine output is mainly distributed to three wheels other than the front wheel located on the outer side of the turning locus.
At this time, although excessive engine output is distributed to the front wheel located on the inner side of the turning locus, a predetermined braking force is imparted to the front wheel located on the inner side of the turning locus by means of the idle-rotation suppression control for the front wheels. With this idle-rotation suppression control, braking forces are applied to the two front wheels, so that the two front wheels become less likely to produce excessive idle rotation. Accordingly, the cornering force generated by means of tires of the two front wheels can be maintained.
Meanwhile, when the two front wheels become less likely to produce excessive idle rotation as a result of application of braking forces to both the front wheels, excessive engine output becomes likely to be distributed to the two rear wheels. In addition, there is no possibility that high engine output is distributed to only one of the two rear wheels (e.g., the rear wheel located on the inner side of the turning locus) because of the idle-rotation suppression control for the rear wheels. Accordingly, the two rear wheels become more likely to produce excessive idle rotations, and as a result, the cornering force generated by means of tires of the two rear wheels becomes likely to decrease.
When the cornering force generated by means of tires of the two rear wheels decreases with the cornering force generated by means of tires of the two front wheels maintained, a yawing moment of the same direction as the vehicle's yawing direction is generated in the vehicle. In other words, there is generated a yawing moment whose direction is opposite the direction of a yawing moment to be generated by means of the above-described vehicle stabilization control.
Accordingly, the over-steer suppressing effect by the vehicle stabilization control cannot be reliably achieved, and as a result, the running stability of the vehicle cannot be maintained. In other words, in the case where the operation of lowering the engine output during execution of the vehicle stabilization control is eliminated in order to secure the running performance and the escaping performance of the vehicle, when the above-mentioned idle-rotation suppression control is also executed during execution of the vehicle stabilization control, there arises a problem that in some cases the running stability of the vehicle cannot be reliably maintained.